U.S. patent application number 12/748229 was filed with the patent office on 2011-09-29 for impedance mediated power delivery for electrosurgery.
Invention is credited to Tim KOSS, Miriam H. Taimisto, Roseanne Varner.
Application Number | 20110238062 12/748229 |
Document ID | / |
Family ID | 44657256 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110238062 |
Kind Code |
A1 |
KOSS; Tim ; et al. |
September 29, 2011 |
Impedance Mediated Power Delivery for Electrosurgery
Abstract
An adaptive algorithm monitors, inter alia, the rate of tissue
impedance change during an electrosurgical procedure. Impedance
levels achieved within a specific timeframe are examined to
determine an impedance ramp and/or slope rate, which indicates the
rate at which a target tissue is undergoing a phase or state change
and, thus, indicates a desired rate of tissue processing. The level
of electrosurgical energy applied to the target tissue is adjusted
in real time in accordance with such rate of impedance change
and/or by predetermined values. Energy is thus applied to the
target tissue at levels that allow tissue phase or state change to
occur in an optimum fashion, for example allowing moisture to
escape from the tissue slowly, and thus avoid thermal damage. As a
result, such undesired results as thermal damage and defective
sealing are mitigated. Another embodiment determines impedance
achieved within a specific interval and adjusts the electrosurgical
energy applied to the tissue after a threshold impedance has been
maintained or exceeded for a predetermined interval. A further
aspect of the invention provides mitigation during processing for
partial tissue coverage of device electrodes and/or for thin
tissue.
Inventors: |
KOSS; Tim; (Discovery Bay,
CA) ; Taimisto; Miriam H.; (San Jose, CA) ;
Varner; Roseanne; (Las Vegas, CA) |
Family ID: |
44657256 |
Appl. No.: |
12/748229 |
Filed: |
March 26, 2010 |
Current U.S.
Class: |
606/34 |
Current CPC
Class: |
A61B 2018/00642
20130101; A61B 18/18 20130101; A61B 18/1445 20130101; A61B
2018/00589 20130101; A61B 2018/0063 20130101; A61B 2018/00875
20130101; A61B 2018/00595 20130101; A61B 2018/00702 20130101; A61B
2018/00678 20130101; A61B 18/1206 20130101 |
Class at
Publication: |
606/34 |
International
Class: |
A61B 18/12 20060101
A61B018/12 |
Claims
1. An electrosurgery method, comprising the steps of: applying
energy to an individual's tissue with an electrosurgical appliance;
monitoring rate of tissue impedance change; generating a signal
indicative of said rate of tissue impedance change; providing a
processor configured to determine an impedance ramp and/or slope
rate from said signal, said impedance ramp and/or slope rate
indicates a rate at which said individual's tissue is undergoing a
phase or state change; said processor configured to continuously
adjust a ramp and/or slope of energy applied to said individual's
tissue via said electrosurgical appliance in rate real time, and to
adjust a rate at which an ultimate level of energy is achieved
while applying energy to said individual's tissue in accordance
with said impedance ramp and/or slope rate; and continuing to
monitor said rate of tissue impedance change and to adjust said
level of energy applied to said individual's tissue until tissue
processing is complete; wherein energy is applied to said
individual's tissue at levels that allow tissue phase or state
change to occur in an optimum fashion.
2. The method of claim 1, further comprising the step of: applying
energy to said individual's tissue at an initial energy level and
increasing said energy level to a terminal energy level.
3. The method of claim 2, further comprising the step of:
increasing said energy level from said initial energy level to said
terminal energy level in any of a series of discrete steps or in a
continuous fashion over time.
4. The method of claim 1, further comprising the steps of:
monitoring tissue impedance; generating a signal indicative of said
tissue impedance; processor configured to determine when a
threshold impedance is reached within a specific interval; said
processor configured to apply a constant, predetermined level of
energy to said individual's tissue after said threshold impedance
is reached and to continue application of said constant,
predetermined level of energy to said individual's tissue for a
predetermined interval; and said processor configured to
discontinue application of energy to said individual's tissue after
completion of said predetermined interval.
5. The method of claim 1, further comprising the step of: said
processor configured to determine if electrosurgical appliance
electrodes are partially covered by the individual's tissue or are
covered by thin tissue by determining if a rate of change and/or
impedance threshold is reached, and thereafter applying a
decreasing ramp rate and/or power cutback to prevent
over-processing of the tissue.
6. An electrosurgery method, comprising the steps of: applying
energy to an individual's tissue with an electrosurgical appliance;
monitoring tissue impedance; generating a signal indicative of said
tissue impedance; providing a processor configured to determine
when a threshold impedance is reached within a specific interval;
said processor configured to apply a constant, predetermined level
of energy to said individual's tissue after said threshold
impedance is reached and to continue application of said constant,
predetermined level of energy to said individual's tissue for a
predetermined interval; and said processor configured to
discontinue application of energy to said individual's tissue after
completion of said predetermined interval.
7. The method of claim 6, said configured to determine when each of
a plurality of threshold impedances is reached within a
corresponding specific interval; and said processor configured to
discontinue application of energy to said individual's tissue after
completion of a last of said corresponding predetermined
intervals.
8. An electrosurgery apparatus, comprising: an electrosurgical
appliance for performing electrosurgery on an individual's tissue;
a source of energy coupled to the electrosurgical appliance by a
control circuit, said control circuit configured to adjust any of
the current and voltage output from said source of energy and,
thus, to adjust power output of said source of energy, said control
circuit configured to adjust said power output of said source of
energy up and/or down in steps and/or in a selected ramp; a sensor
within or proximate to said electrosurgical appliance for
monitoring an effect of said electrosurgical appliance on said
individual's tissue and producing a tissue impedance signal
therefrom; a processor coupled to receive said tissue impedance
signal from said sensor; said processor operating under control of
a program stored in a memory, said processor configured to adjust
the output of said source of energy by issuing control signals to
said source of energy, said processor configured to apply the
signal from said sensor to said program and to adjust the energy
supplied to said individual's tissue by said source of energy in
real time in response to the signal generated by said sensor; and
said processor configured to operate said source of energy to
provide an adaptive power ramp by which a lower level of energy is
initially supplied to the individual's tissue and the output of
source of energy supplied to the individual's tissue is gradually
increased to a higher level of energy, wherein said power ramp is
provided over a predetermined interval during which impedance of
the individual's tissue is monitored in real time and change in
impedance over time is used to determine a slope of a next power
ramp; said processor configured to adjust said power ramp during
each interval of energy application based upon said rate of change
in impedance over time.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The invention relates to electrosurgery. More particularly,
the invention relates to impedance mediated power delivery for
electrosurgery.
[0003] 2. Description of the Prior Art
[0004] The state of the art of electrosurgery is well summarized in
U.S. patent publication no. 2009/0157071 (Wham et al), where it is
stated:
[0005] "Electrosurgery involves application of high radio frequency
electrical current to a surgical site to cut, ablate, or coagulate
tissue. In monopolar electrosurgery, a source or active electrode
delivers radio frequency energy from the electrosurgical generator
to the tissue and a return electrode (e.g., a return pad) carries
the current back to the generator. In monopolar electrosurgery, the
source electrode is typically part of the surgical instrument held
by the surgeon and applied to the tissue to be treated. The patient
return electrode is placed remotely from the active electrode to
carry the current back to the generator.
[0006] In bipolar electrosurgery, one of the electrodes of the
hand-held instrument functions as the active electrode and the
other as the return electrode. The return electrode is placed in
close proximity to the active electrode such that an electrical
circuit is formed between the two electrodes (e.g., electrosurgical
forceps). In this manner, the applied electrical current is limited
to the body tissue positioned between the electrodes. When the
electrodes are sufficiently separated from one another, the
electrical circuit is open and thus inadvertent contact of body
tissue with either of the separated electrodes does not cause
current to flow.
[0007] Bipolar electrosurgery generally involves the use of
forceps. A forceps is a pliers-like instrument which relies on
mechanical action between its jaws to grasp, clamp and constrict
vessels or tissue. So-called "open forceps" are commonly used in
open surgical procedures whereas "endoscopic forceps" or
"laparoscopic forceps" are, as the name implies, used for less
invasive endoscopic surgical procedures. Electrosurgical forceps
(open or endoscopic) utilize mechanical clamping action and
electrical energy to effect hemostasis on the clamped tissue. The
forceps include electrosurgical conductive plates which apply the
electrosurgical energy to the clamped tissue. By controlling the
intensity, frequency and duration of the electrosurgical energy
applied through the conductive plates to the tissue, the surgeon
can coagulate, cauterize and/or seal tissue.
[0008] Tissue or vessel sealing is a process of liquefying
collagen, elastin and ground substances in tissue so that they
reform into a fused mass with significantly-reduced demarcation
between opposing tissue structures. Cauterization involves the use
of heat to destroy tissue and coagulation is a process of
desiccating tissue wherein the tissue cells are ruptured and
dried.
[0009] Tissue sealing procedures involve more than simply
cauterizing or coagulating tissue to create an effective seal; the
procedures involve precise control of a variety of factors. For
example, in order to affect a proper seal in vessels or tissue, it
has been determined that two predominant mechanical parameters must
be accurately controlled: the pressure applied to the tissue; and
the gap distance between the electrodes (i.e., distance between
opposing jaw members or opposing sealing plates). In addition,
electrosurgical energy must be applied to the tissue under
controlled conditions to ensure creation of an effective vessel
seal. Techniques have been developed whereby the energy applied to
the tissue is varied during the tissue sealing process to achieve a
desired tissue impedance trajectory. When a target tissue impedance
threshold is reached, the tissue seal is deemed completed and the
delivery of electrosurgical energy is halted."
[0010] Wham et al takes the approach of incorporating a cooling
period subsequent to a tissue reaction that occurs after the
application of electrosurgical energy to the tissue, where such
electrosurgical energy is applied to the tissue in accordance with
an algorithm that reduces power with increasing tissue impedance
(see Wham et al, FIG. 8). However, this approach merely adjusts the
amount of electrosurgical energy applied as it tracks tissue
impedance vis a vis a target tissue impedance. The approach does
not take in to account the actual change of state within the tissue
and thus does not address such issues as thermal damage to the
tissue and defective sealing.
SUMMARY OF THE INVENTION
[0011] An embodiment of the invention provides an electrosurgical
technique that addresses such issues as thermal damage to the
tissue, partial coverage of the electrodes of the electrosurgical
device by tissue, thin tissue, and defective sealing. This
improvement is accomplished by use of an adaptive algorithm that
monitors, inter alia, the rate of tissue impedance change. An
aspect of the invention thus examines impedance levels achieved
within a specific timeframe to determine an impedance ramp and/or
slope rate, which indicates the rate at which the target tissue is
undergoing a phase or state change and, thus, indicates tissue
processing. The level of electrosurgical energy applied to the
target tissue is adjusted in real time in accordance with such rate
of impedance change. This approach, in effect, applies the energy
at levels that allow tissue phase or state change to occur in an
optimum fashion, for example allowing moisture to escape from the
tissue slowly, and thus avoid thermal damage. As a result, such
undesired results as thermal damage and defective sealing are
mitigated.
[0012] Another embodiment of the invention determines impedance
achieved within a specific interval and adjusts the electrosurgical
energy applied to the tissue after a threshold impedance has been
maintained or exceeded for a predetermined interval.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a block schematic diagram of an apparatus for
impedance mediated power delivery for microsurgery according to the
invention;
[0014] FIG. 2 is a flow diagram showing an algorithm for impedance
mediated power delivery for microsurgery according to a first
embodiment of the invention;
[0015] FIG. 3 is a flow diagram showing an algorithm for impedance
mediated power delivery for microsurgery according to a second
embodiment of the invention;
[0016] FIG. 4 is a timing diagram showing an impedance mediated
power delivery ramp for microsurgery according to the
invention;
[0017] FIG. 5 is a timing diagram showing an impedance mediated
power delivery interval for microsurgery according to the
invention;
[0018] FIG. 6 is a timing diagram showing a modified power delivery
profile according to the invention;
[0019] FIG. 7 is a timing diagram showing an endpoint detection
profile according to the invention; and
[0020] FIG. 8 is a timing diagram showing a partial tissue coverage
mitigation profile according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] An embodiment of the invention provides an electrosurgical
technique that addresses such issues as thermal damage to the
tissue, partial coverage of the electrodes of the electrosurgical
device by tissue, thin tissue, and defective sealing. This
improvement is accomplished by use of an adaptive algorithm that
monitors, inter alia, the rate of tissue impedance change. An
aspect of the invention thus examines impedance levels achieved
within a specific timeframe to determine an impedance ramp and/or
slope rate, which indicates the rate at which the target tissue is
undergoing a phase or state change and, thus, indicates a desired
rate of tissue processing. The level of electrosurgical energy
applied to the target tissue is adjusted in real time in accordance
with such rate of impedance change and/or impedance thresholds
reached. This approach, in effect, applies the energy at levels
that allow tissue phase or state change to occur in an optimum
fashion, for example allowing moisture to escape from the tissue
slowly avoiding thermal damage and/or reducing energy for thin
tissue or partially covered electrodes. As a result, such undesired
results as thermal damage and defective sealing are mitigated.
[0022] Another embodiment of the invention determines impedance
achieved within a specific interval and adjusts the electrosurgical
energy applied to the tissue after a threshold impedance has been
exceeded for a predetermined interval. This approach, in effect,
determines when the tissue phase or state change has successfully
occurred and that the application of energy can be halted.
[0023] FIG. 1 is a block schematic diagram of an apparatus for
impedance mediated power delivery for microsurgery according to the
invention. In FIG. 1, an individual is shown undergoing a procedure
in which electrosurgery is being performed on the individual's
tissue 10 by an electrosurgical appliance 12, as is known in the
art. A source of energy, such as an RF generator 18 is coupled to
the electrosurgical appliance by a control circuit 16. The control
circuit is operable to adjust any of the current and voltage output
and, in some embodiments, adjust the phase relation between the
voltage and current, from the RF generator and, thus, to adjust the
power output of the RF generator. The control circuit can adjust
the RF generator output up and/or down in steps and/or in a
selected ramp and/or slope.
[0024] The effect of the electrosurgical appliance on the tissue is
monitored at the site of tissue treatment by one or more sensors
within or proximate to the electrosurgical appliance. A signal
produced by the one or more sensors is coupled to a sensor circuit
14. The sensors can monitor such factors as temperature, impedance,
RF voltage, RF current, and the like. In the preferred embodiment,
the sensor monitors the components of impedance and RF power.
[0025] The sensor circuit generates an output signal that is
coupled to a processor 15. The processor operates under control of
a program and adjusts the output of the RF generator by issuing
control signals to the control circuit. In doing so, the processor
applies the signal provided by the sensor circuit to the program
and adjusts the RF power supplied to the tissue, for example, in
real time in response to signal generation by the sensors. Thus, in
some embodiments of the invention the process of treating the
tissue is monitored in real time and the effect of the treatment
upon the tissue, as indicated by the sensors, is used to mediate
the application of energy to the tissue. The program may be
retained in a memory 17 and includes both instructions for
operating the processor and parameters that determine how to
respond to signals from the sensor, timing information, and the
like.
[0026] An important feature of the invention is the manner in which
the processor operates the control circuit and, thus, the manner in
which energy is supplied to the tissue, in response to signals
provided to the processor from the one or more sensors via the
sensor circuit. In a preferred embodiment, the one or more sensors
monitor the impedance of the tissue. As the tissue is processed by
application of energy thereto, a phase or state change gradually
occurs and this phase or state change results in a change in the
impedance of the tissue. It is known in the art to monitor tissue
impedance in connection with such treatments. Uniquely, an
embodiment of the invention provides an adaptive power ramp and/or
slope by which a lower level of energy is initially supplied to the
tissue. The output of the RF generator supplied to the tissue is
gradually increased to a higher level of energy and/or the rate of
power output is increased or decreased. This ramp and/or slope is
provided for a predetermined interval. In some embodiments, during
the interval, the impedance of the tissue is monitored in real time
and the change in impedance over time and/or threshold achieved is
used to determine the slope or rate of a next ramp. The change in
impedance is thought to indicate the rate at which tissue phase or
state change is progressing. If the rate of such change occurs too
quickly, the tissue may be degraded as a result of thermal damage,
for example where moisture in the tissue escapes too quickly or
forcefully in the form of steam. Thus, key to the invention is a
recognition that the rate of change of impedance tracks the rate of
phase or state change of the tissue. The processor is programmed to
adjust the energy ramp and/or slope during each interval of energy
application based upon this rate of change in impedance over time
and/or by impedance thresholds achieved. It should be appreciated
that, for purpose of the discussion herein, the ramp of energy
output refers to the difference between the output level at the
start of the ramp and the output level achieved at the end of the
ramp, while the slope refers to the rate at which the energy output
is increased over time.
[0027] One aspect of the invention allows a determination to be
made if the electrosurgical appliance electrodes are partially
covered by the tissue that is being treated, or if the tissue that
is being treated is relatively thin, such as 0.5 mm or less. If the
electrodes are partially covered by the tissue or if thinner tissue
is being treated, the rate of change of impedance is greater
because less tissue is being treated. Accordingly, the energy
supplied or the interval over which energy is supplied can be
adjusted. For example, in some embodiments, if the partial coverage
of tissue or if thinner tissue is being treated, the energy ramp
and/or slope is more gradual, whereas if the tissue is thick, then
the rate of change of impedance is lesser, and the energy ramp
and/or slope is steeper. Other embodiments adjust the power level
and/or interval over which power is delivered to the tissue in
accordance with, for example, rate of change of tissue impedance.
In this way, the invention applies the rate of change in impedance
and/or threshold levels achieved, to mediate energy supplied to the
tissue.
[0028] In an alternate or supplemental embodiment, a target tissue
impedance is established, based upon criteria stored in the memory
and, once that impedance is reached, energy continues to be
supplied for a predetermined interval. That is, a target tissue
impedance is achieved and energy is supplied to the tissue for a
period of time after the impedance is reached. This embodiment of
the invention determines a preferred tissue impedance for
processing and then continues supplying energy to the tissue once
this impedance is reached. This is accomplished by a ramp and/or
slope mechanism similar to that described above, where a measure of
sustained energy is maintained at a particular impedance. When a
certain time has elapsed at this threshold impedance, tissue
processing is considered complete.
[0029] The two embodiments of the invention may be used alone or in
combination. For example, the rate of change in impedance may be
used to determine when sufficient tissue processing has occurred,
that is when a threshold impedance is reached; and the threshold
impedance may then be used to continue processing until the tissue
is completely transformed. In this way, the tissue is processed at
a rate that avoids thermal damage and defective sealing, and the
tissue is processed sufficiently to complete phase or state
change.
[0030] FIG. 2 is a flow diagram showing an algorithm for impedance
mediated power delivery for microsurgery according to a first
embodiment of the invention. In FIG. 2, energy is applied to the
tissue at an initial level (200) to begin tissue processing in a
gentle fashion. The energy level is ramped to a full energy level
(210) in accordance with a ramp and slope that is established as a
function of rate of change of impedance (220). If a threshold
impedance is reached and maintained or exceeded over a
predetermined amount of time, indicating that the tissue is fully
processed (230), the process is complete (250) and energy is no
longer supplied to the tissue. Else, the energy ramp is adjusted
based upon the tissue impedance and the rate of change in the
tissue impedance (240) and the process continues.
[0031] FIG. 3 is a flow diagram showing an algorithm for impedance
mediated power delivery for microsurgery according to a second
embodiment of the invention. In FIG. 3, energy is applied to the
tissue (300) and the tissue impedance is measured (310). If the
threshold impedance is achieved, e.g. 250 Ohms (320), then energy
is applied to the tissue for a predetermined, cumulative interval
t, e.g. 1.5 seconds. At the end of this interval, tissue processing
in complete (340). If the threshold impedance is not achieved, the
tissue impedance is monitored as energy is applied to the tissue
(330) and the process continues. Further, if thin tissue or partial
tissue coverage is detected (350), then the energy level is
reduced, e.g. voltage is reduced by 75% (350), and the process then
continues as outlined above.
[0032] As discussed above, both techniques may be combined. For
example, the application of energy in the embodiment of FIG. 3 may
be in accordance with a ramp and/or slope that is determined as a
function of the rate of change of the tissue impedance and/or an
impedance threshold achieved. Likewise, the interval of energy
application to the tissue in the embodiment of FIG. 2 may be in
accordance with the determination of a threshold impedance, that is
the ramp may be eliminated once the threshold impedance is
achieved, at which point energy is supplied to the tissue at a
higher level.
[0033] FIG. 4 is a timing diagram showing an impedance mediated
power delivery ramp for microsurgery according to the invention. In
FIG. 4, a first ramp 40 is shown over an interval of three seconds.
For purpose of this embodiment of the invention, the ramp interval
is three seconds and the same interval is used for each ramp. Those
skilled in the art will appreciate that other intervals may be used
and that the intervals themselves may be varied as a result of the
rate of impedance change.
[0034] It can be seen that the slope of the first ramp interval
includes a first, steep portion, a shallow middle portion, and a
relatively flat third portion. Thereafter, the energy is reduced
and the next ramp is commenced. In this embodiment, each ramp is
mediated in real time in view of the rate of change of tissue
impedance, and can also include the absolute impedance (as in the
embodiment of FIG. 3) as well. The slope of the second ramp 42
includes less of a steep, initial portion; the slope of the third
ramp 44 has a less pronounced slope; the slope of the fourth ramp
46 has an even shallower slope. The area under each ramp indicates
the total energy supplied to the tissue during the ramp. In the
preferred embodiment, as the tissue is processed and less moisture
is retained in the tissue, the energy can be applied at a greater
rate, thus reducing sealing time. Thus, as the tissue is processed
in this embodiment, more energy is supplied to the tissue, i.e. the
ramp is increased, and the energy is supplied more quickly, i.e.
the slope is increased. In other embodiments, either or both of the
slope and ramp may be increased or decreased at the same time; one
of the slope or ramp may be held constant, while the other of the
slope or ramp is increased or decreased; one of the slope or ramp
may be increased, while the other of the slope or ramp is
increased; or the relative increase and/or decrease of the slope
and/or ramp may be altered over time, all in accordance with the
rate of phase or state change in the tissue. In this way, the rate
of phase or state change in the tissue, as indicated by the rate of
change of tissue impedance, is used to mediate the delivery of
energy to the tissue.
[0035] FIG. 5 is a timing diagram showing an impedance mediated
power delivery interval for microsurgery according to the
invention. In FIG. 5, an initial energy ramp 50 is supplied to the
tissue. A subsequent ramp need not be provided in this embodiment.
Once the desired impedance is reached, the energy supplied to the
tissue 52/54 is maintained at a desired level for a predetermined
interval of time.
EXAMPLES
Modified Power Delivery (Mitigation for Thermal Spread)
[0036] RF energy is delivered to the target tissue in multiple
pulses of energy. The length of each pulse is defined as the RF
Pulse Duration and the maximum number of pulses allowed for each
seal is defined as the Max. RF Pulse Count. See FIG. 6.
Method:
[0037] 1. The first RF pulse for a seal starts at a power level
defined as the RF Setpoint Start Value. See FIG. 6.
[0038] 2. The RF power level is then increased from the RF Setpoint
Start Value by a rate defined as the RF Setpoint Ramp and/or slope
rate until the power level reaches the upper level defined as the
RF Setpoint End Value. The RE power level remains at this value
until the end of the pulse time is reached. See FIG. 6.
[0039] 3. At the end of each pulse, the tissue impedance value is
calculated and recorded as the RF Pulse End Impedance and the power
levels are then set to zero. See FIGS. 6 and 7.
[0040] 4. For all pulses subsequent to the first, the following
evaluations are made. See FIGS. 6 and 7: [0041] If the RF Pulse End
Impedance is less than a threshold defined as RF Setpoint Ramp
Impedance Threshold, the RF power delivered is ramped identical to
the first pulse. [0042] If the RF Pulse End Impedance is greater
than the RF Setpoint Ramp Impedance Threshold, the RF power
delivered is not ramped but stepped directly to the RF Setpoint End
Value.
TABLE-US-00001 [0042] TABLE 1 Typical Values and Ranges - Modified
Power Delivery Value Typical Range RF Pulse Duration 3.0 sec.
0.5-10.0 sec. Max. RF Pulse Count 5 pulses 1-30 pulses RF Setpoint
Start Value 50 watts 25-150 watt RF Setpoint Ramp and/or 50
watt/sec. 1-100 watt/sec. slope rate RF Setpoint End Value 150
watts 50-150 watt RF Pulse End Impedance based on tissue 2-900 ohms
response RF Setpoint Ramp 50 ohms 5-250 ohms Impedance
Threshold
Endpoint Detection
[0043] The sealing cycle is terminated when the tissue impedance
reaches a predetermined threshold for a specified length of time OR
when a fault or error condition is detected. A successful sealing
cycle is defined here.
Method:
[0044] 1. The tissue impedance is calculated using the signals from
the RF monitoring hardware circuits.
[0045] 2. When the calculated tissue impedance exceeds a threshold
level defined as the Impedance Endpoint Threshold, a timer is
started. If the calculated tissue impedance falls below the
Impedance Endpoint Threshold, the timer is halted. See FIG. 7.
[0046] 3. If the above timer accumulates a value defined as the
Seal Endpoint Time, the RF delivery is halted, the user is notified
of the completed seal and the system is placed in the Ready state.
See FIG. 7.
TABLE-US-00002 TABLE 2 Typical Values and Ranges - Endpoint
Detection Value Typical Range Impedance Endpoint Threshold 250 ohms
100-750 ohms Seal Endpoint Time 1.5 sec. 0.1-5.0 secs.
Partial Coverage Mitigation
[0047] The exemplary RF generator should seal tissue that is fully
covered by the RF electrodes, as well as smaller tissue that is
partially covered by the RF electrodes. Partially covered
electrodes can create a challenge to RF delivery due to the
increased rate at which the tissue desiccates. The following
describes the mitigation incorporated in the RF delivery algorithm
to address this issue.
Method:
[0048] 1. The tissue impedance is calculated using the signals from
the RF monitoring hardware circuits.
[0049] 2. When the calculated tissue impedance exceeds a threshold
level defined as the Impedance Cutback Threshold for a duration
defined as the Impedance Cutback Time, the RF delivery is reduced
by decreasing the RF Voltage being delivered. See FIG. 8.
[0050] 3. The RF Voltage is reduced by a value defined as RF
Voltage Cutback.
[0051] 4. If the tissue impedance exceeds the Impedance Cutback
Threshold a second time, the RF Voltage is reduced again by the
value of the RF Voltage Cutback.
TABLE-US-00003 TABLE 3 Typical Values and Ranges - Partial coverage
Mitigation Value Typical Range Impedance Cutback Threshold 700 ohms
100-900 ohms Impedance Cutback Time 0.1 sec. 0.01-2.0 secs. RF
Voltage Cutback 35 volts 1-100 volts
[0052] Although the invention is described herein with reference to
the preferred embodiment, one skilled in the art will readily
appreciate that other applications may be substituted for those set
forth herein without departing from the spirit and scope of the
present invention. Accordingly, the invention should only be
limited by the Claims included below.
* * * * *